In this project we are using the bolus-response of an inert gas in order to determine: the influence of respiratory flow and heartbeat upon the mechanism of airway gas mixing in human subjects; and the influence of branch points and airway constrictions upon longitudinal mixing in physical models of the pulmonary airways. The major finding during the first year of this study was that in the central airways of a normal subject, turbulent flow mixing dominates during inspiration while laminar mixing is dominant during expiration. A possible explanation for this is that the upper airways (the larynx, in particular) produce local turbulence which is propagated far downstream during inspiration while purely laminar flow is more likely to occur during expiration. The recognition of these facts may lead to a better interpretation of existing pulmonary function tests, particularly the so-called wash-out test. Also, the transport of aerosols, which is an important concern in the fields of occupational health and inhalation therapy will certainly be influenced by the presence of turbulent mixing. In the coming year we plan to correlate extent of airway mixing with heart rate in 5 normal subjects. Also, we intend to simultaneously measure the longitudinal distribution of mixing and of turbulence intensity downstream of a tube orifice which has been designed so as to model the hydrodynamic behavior of the larynx.